Temperature in a refrigerated transport container, or another kind of refrigerated storage space, is typically controlled within a temperature range adjacent to a set point or target temperature (in the following referred to as set point temperature, set point or first set point). The refrigerated transport container may for example comprise an insulated enclosure divided in a cooling space and a transport volume. Typically, the transport volume is loaded with perishable product such as meat, vegetables and fruit, etc. The set point temperature is then typically chosen to reduce quality degradation of the perishable product.
The cooling space may e.g. be separated from the transport volume by a panel equipped with one or more openings to allow a return air flow from the transport volume into the cooling space and a supply air temperature flow from the cooling space into the transport volume.
The air flow through the cooling space typically passes at least a return air temperature sensor, a device for reducing the temperature of the passing air, e.g. a cooling unit or system, and a supply air temperature sensor. In such systems, the return air temperature sensor typically measures the temperature of air returning from the transport volume while the supply air temperature sensor measures the temperature of air supplied to the transport volume.
Temperature control protocols may selectively control a cooling unit coupled to the refrigerated transport container in order to maintain temperatures in the transport volume within a temperature range adjacent to the set point temperature.
One typical type of a cooling unit or refrigeration unit used in refrigerated transport containers is based on the so-called vapour compression refrigeration cycle. This cycle comprises at least a compressor, a condenser, an expansion device, an evaporator and a capacity regulating device. The compressor sucks refrigerant vapour from the evaporator and compresses the refrigerant vapour which subsequently flows to the condenser at high pressure. The condenser ejects its heat to a medium outside the refrigerated transport container while condensing the refrigerant vapour. The liquefied refrigerant then flows to the expansion device in which a refrigerant pressure drops. The low pressure refrigerant then flows to the evaporator where the refrigerant evaporates while extracting the required heat from the refrigerated transport container.
Other typical cooling units or refrigeration units used in refrigerated transport containers may be different.
Temperatures in the transport volume are typically unmeasured. In a steady state operation, measured supply air temperature may normally be a fairly accurate representative of a coldest temperature in the transport volume. In the steady state operation, measured return air temperature may usually be a reasonable representative of average temperature in the transport volume. In the steady state operation, a warmest temperature in the transport volume is usually a little higher than return air temperature, but remains unknown and e.g. depends on the way the product is stowed inside the container.
For frozen commodities, typically shipped at set points below −10 to −5° C. and usually around −20° C., it is especially important that product temperature is not too far above set point. Therefore, in frozen operation at set points below −10 to −5° C., it is common practice to control a measured return air temperature closely to the set point.
For chilled commodities, typically shipped at set points above −10 to −5° C., both too high and too low product temperatures are undesirable. The adverse effect of too high above set point is fairly obvious; that is the whole reason why refrigeration is applied. However being too low below set point, chilled commodities may actually suffer as well. Many chilled commodities are susceptible to freezing injury, which especially becomes an issue when sensitive commodities like grapes are shipped at set points just above their freezing point. Some chilled commodities are susceptible to chilling injury, e.g. like bananas turning grey in home fridges. Therefore in chilled mode operation at set points above −10 to −5° C., it is common practice to control a measured supply air temperature closely to the set point.
Traditionally, refrigerated transport containers used to be stuffed with product which was already pre-cooled to a temperature close to set point, so transport volume temperatures were always more or less in the steady state condition.
The current trend however, is that ever more containers are stuffed with warm product right after harvest, whereby it is up to the container's cooling unit to reduce product temperature from stuffing temperature to a temperature range adjacent to the set point temperature. In the banana trade for example, it is now standard operations procedure to load uncooled bananas of around 25° C. in containers operating at a set point of about 13.5° C. In these non-steady state conditions, return air temperature becomes a poor indicator of the warmest temperature inside the transport volume.
Typically, the warmest temperature converges a lot slower to a temperature range adjacent to a set point temperature than return air temperature.
In view of the increasing number of warmly-stuffed containers, there is a need to effectively and efficiently manipulate measured supply and return air temperature in order to ensure that actual transport volume temperatures reside as much as possible and as quickly as possible within a desired temperature range adjacent to a set point temperature, while limiting the risk of inflicting chilling and/or freezing injury at set points above −10° C.